1. Field of the Invention
The present invention relates to a DC offset compensation apparatus and a method thereof.
2. Description of the Background Art
A direct conversion receiver is being actively researched as one receiver structure to recover a signal received by an antenna. The direct conversion receiver converts an received radio frequency signal into a baseband signal at a time without being converted into an intermediate frequency (IF) signal. Thus, the direct conversion receiver can reduce sizes of external devices such as filters and a burden for processing digital signals. Due to these effects, the direct conversion receiver can be lightweight and manufactured at low costs. Also, a system-on-a-chip can be implemented.
FIG. 1 illustrates a simplified block diagram of a typical direct conversion receiver.
The typical direct conversion receiver includes a low noise amplifier (LNA) 101, a mixer 102, a filter 103, a variable gain amplifier 104, and a local oscillator 105, which is also labeled with a reference letter LO.
The LNA 101 is configured to control a gain and amplifies a signal received by an antenna while suppressing noise. The mixer 102 mixes an output signal of the LNA 101 with a signal provided from the local oscillator 105 and outputs a baseband signal. The filter 103 filters an intended signal from the amplified signal. The variable gain amplifier 104 amplifies a signal outputted from the filter 103 while controlling a gain to sustain a power level of a required output signal.
As described above, the typical direct conversion receiver illustrated in FIG. 1 can reduce sizes of external devices such as additional filters and a burden for processing digital signals.
However, the typical direct conversion receiver often has a DC offset, which may be a major cause of deteriorating the performance of the direct conversion receiver.
The DC offset event is generated by local oscillator leakage. Particularly, in the event that a radio frequency signal has a leakage component on an input side of the mixer 102 for a local oscillation signal, and this leakage component is reflected and supplied again as the local oscillation signal. Thus, the radio frequency signal received by the antenna is mixed with its own signal at the mixer 102.
Similarly, in the event that the local oscillation signal has a leakage component on an input side of the mixer 102 for the radio frequency signal, and this leakage component is reflected and inputted to the mixer 102 with the radio frequency signal, the local oscillation signal is mixed with its own signal, resulting in a generation of DC offset at an output terminal of the mixer 102. This type of DC offset is called self-mixing-oriented DC offset. A level of DC offset is not always consistent but continuously variable depending on a power level of a radio frequency signal, a frequency of the radio frequency signal and a frequency of a local oscillation signal.
The DC offset takes place at the output terminal of the mixer 102 due to a mismatch of a load existing at the output terminal of the mixer 102 and a duty error of the local oscillation signal inputted to the mixer 102. The duty error of the local oscillation signal varies depending on the frequency of the local oscillation signal, and thus, the DC offset generated by the duty error of the local oscillation signal varies depending on the frequency of the local oscillation signal.
When devices such as an amplifier and a filter included in a baseband circuit are mismatched with each other, an output signal of the receiver often has a DC offset. This Dc offset varies depending on a cut-off frequency of the filter 103 and a change in a gain of the variable gain amplifier 104.
The direct conversion receiver has a DC offset due to various factors, and a level of DC offset is not always consistent but continuously variable depending on various changes that actually occur while receiving a signal through an antenna. A frequency change in the local oscillation signal, a change in the received signal, a gain change in the amplifier are examples of the changes that cause the continuous variation in DC offset.
If the DC offset is severe, compensation is generally necessary as much as a level of the DC offset. If the DC offset is excessively compensated, another DC offset that is greater than the initial DC offset may be generated, resulting in various limitations that may be more severe than before the compensation is applied.